Three-phase fluid flow in porous rocks during CO2 injection into reservoirs

Injecting carbon dioxide (CO₂) into deep geological formations is part of the process of carbon capture, utilisation and storage (CCUS). When CO₂ is injected into hydrocarbon reservoirs, a double benefit can be achieved. Indeed, injecting CO₂ into oil reservoirs can enhance oil recovery (EOR) and also reduce emissions of this greenhouse gas into the atmosphere. This approach (CCUS-EOR) appears promising as it helps achieve climate goals in a cost-effective manner.
At the same time, CO₂ flooding can complicate the phase behavior of fluids in the reservoir. For example, when hydrocarbons are mixed with CO₂, three-phase liquid-liquid-vapor (LLV) equilibria can occur. This means that the mixture of hydrocarbons and CO₂ is separated to form a vapor and two liquid phases that differ in their physical properties (density, viscosity and phase composition, etc.). These differences affect fluid flow in porous reservoir rocks and the ultimate displacement efficiency in CCUS-EOR projects.
We study the phase behavior of hydrocarbon-carbon dioxide mixtures and the effect of LLV separation on fluid flow using numerical simulation. We show how direct minimization of the Gibbs energy can be used to calculate LLV equilibria, which is a necessary step for subsequent numerical simulation of three-phase transport in porous reservoir rocks.